The implantation or insertion of medical devices into the body of a patient is common in the practice of modern medicine. For instance, in the past ten years stents have emerged as a prime therapy for arthroclerosis because they counteract the effects of intimal hyperplasia from balloon injury. Unfortunately, in-stent restenosis is a disease that may occur from the injury to the vessel wall. Drug eluting stents have a polymeric coating over the stent to release a drug at a prescribed rate for a given duration to counteract the effects of in-stent restenosis. The coating on the stent is in contact with the delivery system (e.g., balloon) along its inner diameter and in contact with the vessel wall along its outer diameter. It is advantageous to optimize the properties of the polymeric coating so as to control the release of drug, have optimum biocompatibility against the vessel wall, and be mechanically compatible with the surface of the balloon. Examples of drug eluting coronary stents include commercially available stents from Boston Scientific Corp. (TAXUS), Johnson & Johnson (CYPHER), and others. See S. V. Ranade et al., Acta Biomater. 2005 January; 1(1): 137-44 and R. Virmani et al., Circulation 2004 February 17, 109(6) 701-5.
Various types of polymeric materials have been used as drug-releasing reservoirs, including, for example, homopolymers such as poly(n-butyl methacrylate) and copolymers such as poly(ethylene-co-vinyl acetate) and poly(isobutylene-co-styrene), for example, poly(styrene-b-isobutylene-b-styrene) triblock copolymers (SIBS), which are described, for instance, in U.S. Pat. No. 6,545,097 to Pinchuk et al. SIBS triblock copolymers have a soft, elastomeric low glass transition temperature (Tg) midblock and hard elevated Tg endblocks. As with many block copolymers, SIBS tends to phase separate, with the elastomeric blocks aggregating to form elastomeric phase domains and the hard blocks aggregating to form hard phase domains. It has been hypothesized that, because each elastomeric block has a hard block at each end, and because different hard blocks within the same triblock copolymer are capable of occupying two different hard phase domains, the hard phase domains become physically crosslinked to one another via the elastomeric blocks. Moreover, because the crosslinks are not covalent in nature, they can be reversed, for example, by dissolving or melting the block copolymer. Consequently, SIBS copolymers are thermoplastic elastomers, in other words, elastomeric (i.e., reversibly deformable) polymers that form physical crosslinks which can be reversed by melting the polymer (or, in the case of SIBS, by dissolving the polymer in a suitable solvent).